Images containing half-tones can be binarized using dither matrices. A dither matrix is a two-dimensional matrix of thresholds. The images are binarized by comparing each successive pixel of a generated image signal to a corresponding threshold of the dither matrix and generating a binary signal, a dithered signal, in dependence upon whether the pixel value of the image signal is greater or smaller than the corresponding threshold in the dither matrix. Therefore, only a single bit is allocated to each pixel in the dithered signal. Further, since adjacent pixels are binarized in response to different thresholds, the local density of the dithered signal will correspond to the density in the original image. A more complete explanation of half-tone binarization using a dither matrix can be found in U.S. Pat. No. 4,766,499 to Inuzuka.
Group III facsimile machines incorporate modified Huffman encoders as recommended by CCITT T.4. Modified Huffman encoding is a one-dimensional coding method in which code words corresponding to black/white run-lengths are generated. The modified Huffman encoding technique used by Group III facsimiles, however, is optimized to the first through eighth characters of the CCITT test chart. Consequently, this technique does not yield an adequate compression when encoding nonstandard images that do not contain characters of the CCITT test chart. Moreover, half-tone images that have been binarized using dither matrices also have a considerably low compression rate when using the modified Huffman encoding method. In some cases, the coded data from half-tone images can even exceed the data in the original image. For example, in a half-tone image on A-4 paper, using a transmission speed of 9600 bits per second (bps), approximately ten minutes are required to transmit the image. Therefore, this conventional technique is inadequate for the transmission of half-tone and nonstandard images.